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Neanderthals who practiced cannibalism may have spread a mad cow-like disease that weakened and reduced populations, thereby contributing to their extinction, according to a new theory.

The theoretical model could resolve the longstanding mystery of what caused Neanderthals, which emerged about 250,000 years ago, to disappear about 30,000 years ago.

"The story of Neanderthal extinction is one of the most intriguing in all of human evolution," says Simon Underdown, a lecturer in the anthropology department at Oxford Brookes University and author of a paper in the journal Medical Hypotheses.

"Why did a large-brained, intelligent hominid that shared so many traits with us disappear?"

To resolve that question, Underdown studied a well-documented tribal group, the Fore of Papua New Guinea, who practiced ritualistic cannibalism and linked that to what could have happened to the Neanderthals.

Evidence of cannibalism

The 100,000-120,000 year-old bones discovered at the cave site of Moula-Guercy near the west bank of the Rhone river suggests a group of Neanderthals defleshed the bones of at least six other individuals and then broke the bones apart with a hammer stone and anvil to remove the marrow and brains.

It's not clear why Neanderthals may have eaten each other. But research on the Fore determined that maternal kin of certain deceased Fore individuals used to dismember corpses and regarded some human flesh as a valuable food source.

Beginning in the early 1900s, anthropologists also began to take note of an affliction among the Fore named kuru. By the 1960s, kuru reached epidemic levels and killed over 1100 people.

Subsequent investigations determined that kuru was related to the Fore's cannibalistic activities and was a form of transmissible spongiform encephalopathy or TSE.

TSEs, of which mad cow disease or bovine spongiform encephalopathy is one, have been around for possibly millions of years, Underdown says.

TSEs cause brain tissue to take on an almost sponge-like appearance, caused by the formation of small holes during the development of the disease.

The disease's latter stages often result in severe mental impairment, loss of speech and an inability to move.

How many could the disease kill?

Underdown created a model, based on the kuru findings, to figure out how the spread of such a disease via cannibalism could reduce a population's size.

For example, he calculated that within a hypothetical group of 15,000 individuals, such a disease could reduce the population to non-viable levels within 250 years.

When added to other pressures, this type of disease could therefore have wiped out the Neanderthals, Underdown believes.

"TSE's could have thinned the population, reducing numbers and contributing to their extinction in combination with other factors [such as climate change and the emergence of modern humans]," he says.

Such diseases have very long incubation periods, he says, so affected individuals may not show symptoms for a very long time.

Similarly, people who eat people with a TSE may not exhibit signs of illness immediately after eating.

"Neanderthals would have been unlikely to spot any causal relationship between cannibalism and TSE symptoms," Underdown says.

Infection via stone tools

Modern clinical tests show that medical instruments can carry infectious prions, which spread TSEs, even after such tools have been sterilised.

So, sharing stone tools could have also spread the disease among Neanderthals, even those that did not practice cannibalism.

"Most scholars now believe that the demise of the Neanderthals was not down to a single causal factor," Barton says.

"However, if genetic studies eventually show that Neanderthals were susceptible to TSE, or other empirical evidence emerges for persistent cannibalism and consumption of brain tissues in late Neanderthal populations, then we may have to rethink our ideas on extinction."